THE CARBOXYL-TERMINAL CXXX SEQUENCE OF GI-ALPHA, BUT NOT RAB5 OR RAB11, SUPPORTS RAS PROCESSING AND TRANSFORMING ACTIVITY

Although the heterotrimeric G(i)alpha subunit terminates in an apparen t CXXX prenylation signal (CGLF), it is not modified by isoprenylation . To determine if the G(i)alpha CXXX sequence can signal prenylation w hen placed at the carboxyl termini of normally prenylated proteins, we have characterized the processing and biological activity of chimeric oncogenic Ras proteins that terminate in the G(i)alpha CXXX sequence (Ras/G(i)alpha). Surprisingly, these chimeras were prenylated both in vivo and in vitro, demonstrated significant membrane association, exhi bited transforming activity, and induced transcriptional transactivati on from Ras-responsive elements. We then extended these studies to det ermine if, unlike the CC or CXC carboxyl-terminal sequences of other R ab proteins, the carboxyl-terminal CXXX sequences of the Ras-related R ab5 and Rab11 proteins represent conventional CXXX prenylation signals that can support Ras processing and transforming activity. Unexpected ly, these Ras/Rab chimeras were nonprenylated, were cytosolic, and lac ked detectable transforming or transcriptional transactivation activit y. Taken together, these results suggest that the context within which a CXXX sequence occurs may also critically control the modification o f a protein by prenylation, and that the Rab5 and Rab11 carboxyl termi ni do not possess conventional CXXX sequences. Instead, their CCXX and CCXXX motifs may represent additional classes of protein prenylation signals.